It is known in the art that a pretension device of a seat belt and/or air-bag system for a rear crash is operated when the rear vehicle crashes against the front vehicle, as disclosed in Japanese Patent Publications Nos. 2002-283949 and 2004-058850.
In the prior art system, a rear crash has been detected by one acceleration sensor assembled in an electronic control unit (ECU). And therefore, the rear crash may be erroneously detected and thereby an erroneous operation of the pretension device and/or the rear crash air-bag may happen to occur.